Abstract
The analysis of large molecular line surveys of the Galactic plane is essential for our understanding of the gas kinematics on Galactic scales and, in particular, its link with the formation and evolution of dense structures in the interstellar medium. An approximation of the emission peaks with Gaussian functions allows for an efficient and straightforward extraction of useful physical information contained in the shape and Doppler-shifted frequency of the emission lines contained in these enormous data sets. In this work, we present an overview and the first results of a Gaussian decomposition of the entire Galactic Ring Survey (GRS)13CO (1–0) data that consists of about 2.3 million spectra. We performed the decomposition with the fully automated GAUSSPY+ algorithm and fitted about 4.6 million Gaussian components to the GRS spectra. These decomposition results enable novel and unexplored ways to interpret and study the gas velocity structure. We discuss the statistics of the fit components and relations between the fitted intensities, velocity centroids, and velocity dispersions. We find that the magnitude of the velocity dispersion values increase towards the inner Galaxy and around the Galactic midplane, which we speculate is partly due to the influence of the Galactic bar and regions with higher non-thermal motions located in the midplane, respectively. We also used our decomposition results to infer global properties of the gas emission and find that the number of fit components used per spectrum is indicative of the amount of structure along the line of sight. We find that the emission lines from regions located on the far side of the Galaxy show increased velocity dispersion values, which are likely due to beam averaging effects. We demonstrate how this trend has the potential to aid in characterising Galactic structure by disentangling emission that belongs to the nearby Aquila Rift molecular cloud from emission that is more likely associated with the Perseus and Outer spiral arms. With this work, we also make our entire decomposition results available.
Highlights
The study of the velocity structure of molecular gas is of vital importance for understanding the origin and evolution of structures in the interstellar medium (ISM)
The Carbon monoxyde (CO) emission lines contain useful velocity information: The Doppler shift of the centroid frequency of the line, compared to the rest frequency in the local standard of rest (LSR), comes from the bulk motion of the gas, which is mostly due to Galactic dynamics; and the width of the line beyond the natural line broadening is caused by thermal and non-thermal contributions, Full Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. u-strasbg.fr/viz-bin/cat/J/A+A/633/A14
We considered whether the ∼0.1% of Galactic Ring Survey (GRS) spectra with bestfit solutions that use a high number of components (Ncomp > 10) are connected with very complex lines of sight or whether they result from artefacts or problems in the decomposition
Summary
The study of the velocity structure of molecular gas is of vital importance for understanding the origin and evolution of structures in the interstellar medium (ISM). We currently still lack a study of the detailed velocity structure of the molecular gas on Galactic scales, with sufficient spatial resolution to resolve the inner structure of molecular clouds ( 1 ) and sufficient spectral resolution to resolve the thermal linewidth of the cold molecular gas (∼0.2 km s−1) Such a study requires analysing the entire data set of one of the more recent large Galactic plane surveys in its native spatial and spectral resolution. In a forthcoming work (Riener et al, in prep.), we aim to present distance estimates to the Gaussian fit components This will enable discussion about the Galactic distribution of the gas emission and variations with Galactocentric distance of the kinematic properties of the gas, and this will foster synergies with other ISM tracers, such as the upcoming large-scale dust extinction map from the PROMISE project (Kainulainen et al, in prep.)
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